1. Field of the Invention
The present invention generally relates to a developer feeding member for feeding a developer and also to a developing device having this developer feeding member. More specifically, the present invention is directed to such a developer feeding member suitably used in an electrophotographic image forming apparatus and a process cartridge.
2. Related Background Art
In typical developing devices of image forming apparatuses such as copying machines, generally speaking, two sets of typical developer feeding members are illustratively shown in FIGS. 10A and 10B and FIGS. 11A and 11B. It should be noted that FIG. 10A and FIG. 11A are diagrams showing the developer feeding member as a single body, which is viewed from a substantially front surface, whereas FIG. 10B and FIG. 10B are cross-sectional diagrams showing the developer feeding members arranged in developer containers, which is viewed from the side direction.
The developer feeding members 1 and 11 are arranged within developer containers 2 and 12 that the developer is stored therein (the developer will be optionally referred to as "toner" hereinafter). Since these developer feeding members 1 and 11 are rotated along the arrow directions, these developer feeding members 1 and 11 may feed the developer toward developing areas (not shown) in each of which a developing sleeve and a regulating blade are provided in a left side of the corresponding drawing. Also, these developer feeding members 1 and 11 may agitate the developer to crumb it at the same time when the developer is fed.
The developer feeding member 1 constructed by bending a rod member in a crank shape and shown in FIG. 10A does not have a leave high toner feeding ability. This developer feeding member 1 is usually used in the developer container 2 having such a shape that, as represented in FIG. 10B, toner contained inside this developer container may be relatively easily fed to the developing area, namely the developer container 2 having the shape capable of feeding a certain amount of toner with the aid of only the gravity.
On the other hand, the developer feeding member 11 shown in FIG. 11A has a very a superior toner feeding ability. This developer feeding member 11 is structured in such a manner that a proximal end "A" of an elastic sheet 11g is fixed on a rigid body 11a of a rotational center portion. Since the developer feeding member 11 has very superior toner feeding ability, this developer feeding member 11 is usually employed in another developer container 12 having such a shape (see FIG. 11B) by which the toner contained in this developer container 12 cannot be easily fed to the developing area, namely the developer container 12 having the shape, by which this toner can be hardly fed with the aid of only the gravity. In this case, since the toner must be positively fed, an elastic sheet having a relatively high flexural rigidity is frequently used.
The developer container 12 of FIG. 11B may have a merit in that the weight of the toner itself contained in the developer container 12 is not given to the developing area. However, the dimension of this developer container 12 increases, thereby being not so popularly used in recent years.
However, for instance, in laser beam printers for producing images using laser beams, there is a trend that an image density is decreased under an initial use state of developing units (in such a state that much toner exists in the developer container), while the needs of making "fine particles of the developer" and of increasing "process speed" are emphasized so as to realize high image qualities for improving 1-dot reproducibility.
The major cause of lowering of this image density could be investigated by the inventors. That is, in the initial use state, there is a trend that the toner having a relatively small particle diameter contained in a developer container is collected to places in the vicinity of a developing sleeve. As a result, triboelectricity distribution of the toner coated on this developing sleeve is broadened (namely, a ratio of the toner having optimum triboelectricity to be developed is reduced). As a result, the developing ability is lowered. Thus, existence of the toner having small particle diameter may cause a problem, generally speaking, the smaller average particle diameter of the toner becomes, the more the above-described phenomenon is emphasized not only under the initial use state.
It was also confirmed that in the case where such patterns in which small amount of toner is consumed, are continuously printed out, this phenomenon is emphasized. For instance, the density of a solid black image becomes lower, which is printed out immediately after some blank copy images are continuously printed out. This is because the smaller the toner consumption amount becomes, the larger the fine particle amount present in the vicinity of the developing sleeve is increased.
Furthermore, it was confirmed that this phenomenon is further emphasized when a process speed is increased. This is considered such that developing time per unit area is shortened because of the increased process speed. Even in the case where such trends are more enhanced that "the particle diameter of he developer is made as small" and "process speed is made to be high", as a first solution idea capable of solving such a problem of density reduction, the diameters of particles of the toner are made uniform (namely, fine particles of the toner are excluded upon manufacturing). However, this solution will cause various difficulties. That is, the yield achieved when the toner is manufactured will be very lowered, and higher manufacturing cost will be required, which never constitutes a realistic solution.
As a consequence, in order to avoid such a state that fine particles contained in the toner stored in the developer container are collected to the places in the vicinity of the developing sleeve, the following method may be conceived. That is, a large amount of fresh toner contained in the developer container is not fed to the places in the vicinity of the developing sleeve within the developing area.
As one of these realistic solution methods, there is such a method for lowering the toner feeding ability to the developing area.
When the toner feeding ability is lowered, it was recognized that the above-explained problem could be solved. In contrast, however, there arises such a problem in that even though the toner remains within the developing container, there is no toner in the vicinity of the developing sleeve within the developing area, which causes "follow characters (edge effects)" in an image, and a sufficiently large amount of toner could not be fed to the developing area. As a result, the toner remaining amount in the vicinity of the developing area becomes uneven state in a longitudinal direction (namely axial direction of the developing sleeve), for example, much toner is left at a center portion, whereas less toner is left at both end portions.
When such an uneven state occurs, even if a sufficient amount of the toner is left in the developer container, since the toner remaining amount is locally small, the following problems will occur. That is to say, the quality of the toner located in the vicinity of the developing sleeve and the regulating blade in the places of the small amount toner are deteriorated. As a result, such defective images are readily produced that the image density is lowered due to toner deterioration, and the "follow characters (edge effects)" caused by toner depletion occur.
As previously described, when "feeding ability" is weakened, there is a merit in that the lowering of the density of image at "an initial use state" can be prevented from occurring. However, there is a possibility that when "feeding ability" is weakened, the defective images are produced under "substantially completely used state" (in other words, when an amount of developer contained in the developer container is small).
To avoid such a problem, the following methods may be conceived. That is, the toner in the developer container is continuously fed to the developing area by a user, and/or the developer container itself is vibrated by the user in order to avoid such a condition that the toner remaining amount becomes uneven in the longitudinal direction. However, these solution methods are not realistic methods in view of user ability.
As apparent from the foregoing descriptions, the following functions of a developer feeding member are desirably expected. That is to say, this developer feeding member may have lower "feeding ability" under an "initial use state", and furthermore, may have higher "feeding ability" under "substantially completely used state".